Chromium raises vacancy concentration and promotes grain-boundary Al segregation in Ni–Cr–Al

• Bulk: Chromium lowers the vacancy formation energy by 0.28 eV (Ni–4Al → Ni–15Cr–4Al), raising the equilibrium vacancy fraction by 11 × at 1273 K (1000 ∘ C) and 54 × at 753 K (480 ∘ C); vacancy migration barriers are essentially unchanged. • Bulk: Because the increase is in vacancy concentration rather than mobility, the effective diffusion coefficients in Ni–8Cr–4Al are significantly higher than in Ni–4Al or Ni–8Cr. • Grain boundaries: Adding Cr consistently promotes Al enrichment at GBs to 1.5 × the bulk level, while Cr itself shows little segregation. • Grain boundaries: For segregation, the presence of Cr matters more than its exact level; no systematic increase in Al enrichment is observed with higher Cr above this threshold. • GB character dependence: Al segregation is orientation dependent–High angle Σ5 and Σ11 enrich more strongly than Σ3 (111 twin) GB segregation morphology: Resulting Al-rich GB core with a surrounding Cr-enriched zone mirrors observed intergranular oxide morphologies. Chromium additions are known to promote the formation of a protective Al oxide, alumina, in Ni-Cr-Al alloys, a phenomenon known as the third-element effect (TEE). Using atomistic simulations, we show that Cr lowers vacancy formation energies while leaving migration barriers largely unchanged. This reduction in formation energy leads to a strong amplification of equilibrium vacancy concentrations: compared to Ni–4Al, the concentration in Ni–15Cr–4Al is 11 times higher at 1000 ∘ C and more than 54 times higher at 480 ∘ C. When combined with tracer mobilities, this produces effective diffusion coefficients up to an order of magnitude greater in the ternary alloy. In parallel, Cr promotes aluminum enrichment at grain boundaries, raising Al levels to about one and a half times the bulk concentration across the boundaries studied. These metal-phase mechanisms provide a quantitative route by which Cr facilitates rapid Al delivery to oxidation fronts, complementing oxygen-gettering models and helping explain the TEE, at both high and intermediate homologous temperatures.